System and method for applying smoothed beamforming

ABSTRACT

A method in a wireless station in communication with an access point over a communication channel includes receiving a channel estimation matrix; determining a beamforming matrix using the channel estimation matrix, the beamforming matrix being generated for a plurality of down-sampled feedback indices; the beamforming matrix including beamforming weights for one or more antennas associated with the access point, each beamforming weight being a complex value including an amplitude and a phase; setting the phase of the beamforming weight for a last antenna associated with the last feedback index of the beamforming matrix to zero and rotating the phase of each beamforming weight for each of the other antennas; performing smoothing of the beamforming matrix for each feedback index by normalizing each column vector of the beamforming matrix; compressing the smoothed beamforming matrix; and providing the smoothed beamforming matrix as output.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/731,702, entitled SYSTEM AND METHOD APPLYING SMOOTHED BEAMFORMINGALGORITHM, filed Sep. 14, 2018, which is incorporated herein byreference for all purposes.

FIELD OF THE DISCLOSURE

The present disclosure relates to wireless communication and, inparticular, to a system and method for generating smoothed beamformingfeedback matrix at a beamformee for feedback to an access point.

BACKGROUND OF THE DISCLOSURE

The IEEE 802.11ac wireless local area network (WLAN) standard is used inimplementing very high throughput (VHT) WLANs. In VHT-WLAN, 256-QAMmodulation, the extension of bandwidth up to 160 MHz, and multiple-inputmultiple-output (MIMO) transmission up to 8 streams are defined toprovide 6.9 Gbps data rate. Furthermore, the IEEE 802.11n and beyond(e.g. IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ax) wireless localarea network (WLAN) standard has adopted beamforming (BF) schemes toimprove spectral efficiency and throughput with multiple antennas. Todesign the transmit beam, a channel sounding process to feedback channelstate information (CSI) is implemented. The beamformer uses the channelstate information to precisely steer the transmitter's transmittedenergy toward the receiver.

More specifically, the IEEE 802.11ac WLAN standard includes closed-loopbeamforming (BF) schemes for single-user beamforming (SU-BF) methods andmultiuser beamforming (MU-BF) methods to improve spectral efficiencywith a given channel condition. For these beamforming transmissions, anaccess point (AP) sends a sounding packet including only preambles andreceives a compressed beamforming frame with modified downlink channelinformation from the stations (STAs). For instance, with the beamformingthus applied, in MU-BF mode, an access point can transmit multiple datastreams simultaneously to multiple stations.

SUMMARY OF THE DISCLOSURE

The present disclosure discloses a device and method for controlling aconnected device in a mobile device, substantially as shown in and/ordescribed below, for example in connection with at least one of thefigures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

In embodiments of the present disclosure, a method in a wireless stationdeployed in a wireless local area network and in communication with anaccess point over a communication channel, the access point transmittingusing a plurality of antennas includes receiving a channel estimationmatrix characterizing the communication channel, the channel estimationmatrix being associated with a plurality of feedback indices relating torespective plurality of subcarriers; determining a beamforming matrixusing the channel estimation matrix, the beamforming matrix beinggenerated for a plurality of down-sampled feedback indices being asubset of the plurality of feedback indices; the beamforming matrixincluding beamforming weights for one or more antennas associated withthe access point, each beamforming weight being a complex valueincluding an amplitude and a phase; setting the phase of the beamformingweight for a last antenna associated with the last row index of thebeamforming matrix to zero and rotating the phase of each beamformingweight for each of the other antennas; performing smoothing of thebeamforming matrix for each feedback index by normalizing each columnvector of the beamforming matrix; compressing the smoothed beamformingmatrix; and providing the smoothed beamforming matrix as output.

In embodiments of the present disclosure, a wireless station deployed ina wireless local area network and in communication with an access pointover a communication channel, the access point transmitting using aplurality of antennas, the wireless station including a processor; and amemory coupled with the processor, wherein the memory is configured toprovide the processor with instructions which when executed cause theprocessor to: receive a channel estimation matrix characterizing thecommunication channel, the channel estimation matrix being associatedwith a plurality of feedback indices relating to respective plurality ofsubcarriers; determine a beamforming matrix using the channel estimationmatrix, the beamforming matrix being generated for a plurality ofdown-sampled feedback indices being a subset of the plurality offeedback indices; the beamforming matrix including beamforming weightsfor one or more antennas associated with the access point, eachbeamforming weight being a complex value including an amplitude and aphase; set the phase of the beamforming weight for a last antennaassociated with the last row index of the beamforming matrix to zero androtating the phase of each beamforming weight for each of the otherantennas; perform smoothing of the beamforming matrix for each feedbackindex by normalizing each column vector of the beamforming matrix;compress the smoothed beamforming matrix; and provide the smoothedbeamforming matrix as output.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are disclosed in thefollowing detailed description and the accompanying drawings.

FIG. 1 is a system diagram illustrating an environment in which thebeamforming system and method of the present disclosure can be appliedin some examples.

FIG. 2 is a schematic diagram of a wireless station in some examples.

FIG. 3 is a flowchart illustrating the beamforming matrix generationmethod implemented in a beamformee in embodiments of the presentdisclosure.

FIG. 4 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments.

FIG. 5 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments.

FIG. 6 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments.

FIG. 7 illustrate a pseudocode which can be used to implement the vectorswapping operation in some examples.

DETAILED DESCRIPTION

According to embodiments of the present disclosure, a beamforming matrixgeneration system and method is implemented at a beamformee to generatea smoothed beamforming feedback matrix which can be fed back to abeamformer for performing beamforming on the transmitted beams. In someembodiments, the beamforming matrix generation system and method obtainsa beamforming feedback vector or matrix based on a beamforming algorithmand applying down-sampled subcarrier feedback indices. The beamformingmatrix generation system and method may further perform vector swapping,set last antenna's phase equals to zero while rotating other antennas'phase accordingly, apply smoothing and compression. The beamformingmatrix generation system and method of the present disclosure addressesperformance limitation issues of conventional wireless systems, such assmall power issue, quantization issue, and vector swapping issue.

In some embodiments, the beamforming matrix generation system and methodof the present disclosure is implemented in a wireless station toprovide the medium access controller of an access point with compressedbeamforming matrices and signal-to-noise ratio (SNR) estimates forspecific data subcarriers or subcarrier groups to support beamformingfeedback. In one example, the beamforming matrix generation system andmethod of the present disclosure is implemented in wirelesscommunication systems constructed under the IEEE 802.11n standard andbeyond. Furthermore, the beamforming matrix generation system and methodof the present disclosure can be applied to support both single-usermultiple-input multiple-output (SU-MIMO) and multi-user multiple-inputmultiple-output (MU-MIMO) Wi-Fi operation. MIMO wireless systems improvedata rate by transmitting multiple spatial streams in parallel. The datarate can be further enhanced by using orthogonal frequency divisionmultiplexing (OFDM) or similar techniques within each spatial stream.

MIMO communication systems often employ beamforming to improve receiverperformance. Beamforming operates by directing or steering the transmitbeam to the receiver to improve the receive power and the signal tonoise ratio (SNR) at the receiver. In particular, beamforming changesthe directionality of the antennas (transmit and/or receive antennas) toachieve spatial selectivity and thereby improves the receive power andthe SNR at the receiver. Beamforming is sometimes referred as beamsteering. In general, any device that shapes its transmitted frames iscalled a beamformer and a receiver of these frames is called abeamformee.

System Overview

FIG. 1 is a system diagram illustrating an environment in which thebeamforming system and method of the present disclosure can be appliedin some examples. Referring to FIG. 1, a wireless system 10, which canbe a wireless local area network (WLAN) implemented using any of theIEEE 802.11 standard, includes an access point (AP) 12 with N_(tx)number of antennas and one or more wireless stations (STAs) 14, eachequipped with one or more antennas. In the present illustration, thewireless system 10 includes M wireless stations, each wireless stationequipped with respective N_(rx(b)) number of antennas where b is 1 ormore, b denotes the wireless stations and has a value of 1 to M. Inparticular, the wireless stations may have the same number of antennasor the wireless stations may each have a different number of antennas.It is not necessary that all the wireless stations have the same numberof antennas. In some cases, the wireless stations are implemented usinga single antenna due to space and cost considerations. In SU-BF(single-user beamforming) mode, the AP 12 sends data packets to one STA14 after channel sounding. In MU-BF (multi-user beamforming), the AP 12transmits data packets with multiple destination STAs 14 after receivingthe channel state information from the multiple wireless stations.

More specifically, IEEE 802.11 standards specify explicit beamformingwhere a channel sounding process is used to receive channel stateinformation (CSI) feedback from a receiver to a transmitter. Forinstance, the access point 12 sends a sounding packet, also referred toas a null data packet (NDP), including only preambles and no data to awireless station 14. The wireless station 14, upon receiving the nulldata packet, performs channel estimation and feedback compressed channelstate information to the access point 12. The access point (thetransmitter) receives the compressed beamforming frame including thechannel state information and uses the information to shapes itstransmitted beam so as to precisely steer the AP's transmitter'stransmitted energy toward the wireless station (the receiver).

To improve channel estimation performance, wireless receivers sometimesperform channel smoothing on the estimated channel. In the presentdescription, a wireless channel is said to be “smooth” when thesubcarrier channel coefficients are correlated across frequency. Thatis, the frequency response of the wireless channel has nodiscontinuities or sudden changes. If the channel is smooth,interpolation-based channel estimation can be applied to improve thechannel estimation performance.

Wireless channel is itself smooth. However, in some cases, such as whenthe beamforming is applied, the smoothness of the channel can be broken.Performing channel smoothing on a channel with broken smoothness canactually degrade the channel estimation. Accordingly, under IEEE801.11standards, the access point (as the beamformer) indicates explicitly tothe wireless station (as the beamformee) whether a transmitted datapacket has been beamformed or whether the wireless station shouldperform channel smoothing. The wireless station can thus determine if itshould apply channel smoothing during the channel estimation operationbased on the indication from the beamformer. Channel estimationperformance may be degraded when channel smoothing is not applied.

In embodiments of the present disclosure, the beamformee generates thebeamforming feedback vector or matrix by applying a beamformingalgorithm to the channel estimation matrix. Various beamformingalgorithms are known in the art to generate the beamforming vector ormatrix from the channel estimation matrix. In some examples, thebeamforming matrix generation system and method may use singular valuedecomposition (SVD) to generate the beamforming vector/matrix. In otherexamples, the beamforming matrix generation system and method may usegeometric mean decomposition (GMD). Other beamforming algorithms,presently known or to be developed, may also be used in other examplesof the present disclosure.

In the present description, the beamforming feedback matrix may bereferred to as a beamforming matrix and typically includes one or morecolumn vectors, also referred to as feedback vectors, beamformingfeedback vectors, or beamforming vectors. It is understood that a vectorrefers to a single row or a single column of a matrix. In the followingdescription, references to a “beamforming matrix” may include abeamforming matrix or a beamforming vector, or one or more columnvectors forming the beamforming matrix.

Factors that break the smoothness of a beamformed channel includeeigenvector subspace swapping, phase discontinuity caused by beamformingalgorithm, and quantization error.

(1) Eigenvector Subspace Swapping

In eigenmode beamforming, the beamforming weights are the right singularvectors V_(k) of the channel H_(k), where V_(k) denotes the J singularvectors corresponding to the J largest singular values of the channelH_(k). In some cases, arranging the beamforming weights from the largestsingular values to the smallest may result in vector swapping betweenadjacent subcarriers.

For example, in the case of two subcarriers SC1 and SC2 each feedingback two feedback vectors V1 and V2. In the conventional beamformingmethod, for each subcarrier, the vectors are arranged so that the vectorwith the highest singular value is listed first. In this case,subcarrier SC1 may end up with a feedback matrix of [V1, V2] whilesubcarrier SC2 may end up with feedback matrix of [V2, V1]. When thepositions of the feedback vectors are swapped, the channel smoothnesscan become broken.

(2) Phase Discontinuity Caused by Beamforming Algorithm

A beamforming weight is a complex weight represented by the amplitudeand the phase shift to be applied for each antenna. One factor thatcontributes to the broken smoothness is phase discontinuity. UnderIEEE802.11 standard, a column-wise phase shift is to be applied to theas-computed beamforming matrix to make the last antenna's phase equal tozero and the weight equal to non-negative real numbers.

When the last antenna's beamforming weight is small, phase discontinuitycan result. For example, assume the number of transmit antenna at abeamformer is 2 (N_(tx)=2) and number of receive antenna at a beamformeeis 1 (N_(rx)=1), then the frequency channel response H may be expressedas H=[|h₁₁|e^(jθ) ¹ |h₁₂|e^(jθ) ² ], and the corresponding beamformingvector V is given by:

$V = {H^{*} = {\begin{bmatrix}{{h_{11}}e^{{{- j}\;\theta_{1}}\;}} \\{{h_{12}}e^{{{- j}\;\theta_{2}}\;}}\end{bmatrix} = {\underset{\underset{D}{︸}}{e^{{{- j}\;\theta_{2}}\;}}\underset{\underset{\overset{\sim}{v}}{︸}}{\begin{bmatrix}e^{j{({\theta_{2} - \theta_{1}})}} & 0 \\0 & 1\end{bmatrix}\begin{bmatrix}{h_{11}} \\{h_{12}}\end{bmatrix}}}}}$

When the last row of the beamforming vector V, that is, the lasttransmit antenna's beamforming weight, is too small, ∥|h₁₂|e^(jθ) ² ∥≈0,then sudden change of the phase (θ₂) occurs between subcarriers. Forexample, suppose the following two possible channel estimation resultsare obtained: ε and −ε where ε is small number. Even though differencebetween these two values are very small, the phase of these two valuesare totally different (180°). As a result, channel smoothness is brokenin |h₁₁|e^(j(θ) ¹ ^(θ) ² ⁾.

When beamformee feeds back channel station information including θ₂,this problem can be resolved. However, when the number of receiveantennas at the beamformee is more than 1 (N_(rx)>1), singular valuedecomposition (SVD) is used to obtain the beamforming matrix/vector.Since singular vector is rotational invariant, SVD algorithm may providee^(jθ)v with arbitrary θ in different subcarrier. In this case, feedingback θ₂ does not help.

(3) Quantization Error

Quantization or compression of the beamforming matrix can also result inbroken smoothness in the channel. For example, before quantization, aset of beamforming weights may have both acceptable correlation andphase angle values. However, after quantization, the phase angle canbecome quite high while the correlation is closed to 1. This can meanthat all of antenna's weight is rotated to a certain degree which breakschannel smoothness.

Embodiments of the beamforming matrix generation system and method ofthe present disclosure overcome the drawbacks of the conventionalsolutions.

Device Overview

FIG. 2 is a schematic diagram of a wireless station in some examples.FIG. 2 illustrates an exemplary configuration of a wireless station insome embodiments. It is understood that FIG. 2 is only representative ofa generic wireless station and that in actual implementations, thewireless station may use various configurations and may include otherelements not shown in FIG. 2. Referring to FIG. 2, a wireless station(STA) 50 includes one or more antennas 53 coupled to a radio frequency(RF) front end 52. A receiver circuit 54 and a transmitter circuit 56are coupled to the RF front end 52 to receive signals from and transmitsignals to the antennas 53.

The wireless station 50 includes a processor 60 for controlling theoperation of the wireless station. The processor 60 executesinstructions stored therein to perform various operations, includingbeamforming matrix generation. The processor 60 is in communication witha system bus 58. Through the system bus 58, the processor 60 is incommunication with one or more system components of the wireless station50. For example, the wireless station 50 may include a memory 62 forstoring instructions and data, a display 64 and an I/O interface 66 forinterfacing with a user or for providing status indication.

Beamforming Overview

In embodiments of the present disclosure, the beamformee generates thebeamforming feedback matrix and feedback to the beamformer thebeamforming feedback matrix for each subcarrier and other information asthe channel state information. In some embodiments, a wireless stationis the beamformee configured to generate a compressed beamforming matrixand feed back the compressed beamforming matrix to an access point asthe beamformer. The access point, as the beamformer, receives thecompressed beamforming matrix and reconstructs the beamforming matrix soas to perform beamforming or beam steering. That is, the access pointmodifies its antennas so as to steer the transmit beam towards thewireless station based on the beamforming matrix received from thewireless station.

FIG. 3 is a flowchart illustrating the beamforming matrix generationmethod implemented in a beamformee in embodiments of the presentdisclosure. In some embodiments, the beamformee is a wireless station ina WLAN in communication with an access point as the beamformer.Referring to FIG. 3, a beamforming matrix generation method 100implemented in the beamformee receives a null data packet including apreamble and no data (102). The method 100 performs channel estimationby computing a channel estimation matrix H (104). The channel estimationmatrix H has N_(tx) number of columns, corresponding to N_(tx) number oftransmit antennas at the beamformer, and N_(rx) number of rows,corresponding to N_(rx) number of receive antennas at the beamformee.

With the channel estimation matrix H thus determined, the method 100computes the beamforming matrix using a beamforming algorithm (108). Inone embodiment, the beamforming matrix is generated by decomposing thechannel estimation matrix H using singular value decomposition (SVD). Inanother embodiment, the beamforming matrix is generated by decomposingthe channel estimation matrix H using geometric mean decomposition(GMD). The method 100 perform smoothing to generate a smoothedbeamforming matrix V (110). The smoothed beamforming matrix V may bestored in the wireless station and transmitted to the beamformer as thebeamforming feedback signal (112). In this manner, the beamformingmatrix generating method 100 generates the beamforming feedback matrix Vat the beamformee to be fed back to the beamformer to performbeamforming.

Smoothed Beamforming Matrix Computation Overview

FIG. 4 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments. In particular, FIG. 4illustrates in more details the smoothed beamforming matrix computationsteps implemented at the beamformee in the method 100 of FIG. 3 in someembodiments. Referring to FIG. 4, a smoothed beamforming matrixcomputation method 150 receives the channel estimation matrix H as aninput. The channel estimation matrix H may or may not have been smoothedby channel smoothing. The method 150 obtains a beamforming matrix orvector using a beamforming algorithm, such as SVD or GMD, and usingdown-sampled subcarrier feedback indices (152). For example, when SVD isused as the beamforming algorithm, the beamforming feedback matrix isformed by the right singular vector of the channel estimation matrix H.Furthermore, in the present embodiment, the beamforming matrix is formedby down-sampling the subcarriers of the channel, as will be explained inmore detail below.

The method 150 then performs smoothing on the beamforming feedbackmatrix thus obtained (154). The method 150 then quantize the beamformingfeedback matrix (156). The method 150 thus generates a compressed andsmoothed beamforming matrix V (158).

In some embodiments, the method 150 may further analyze the compressedbeamforming matrix V and determine a channel smoothing indicator for thechannel estimation operation (160). The method 150 generates locally achannel smoothing indicator “CHE Smoothing” which is fed back to thechannel estimation operation to instruct the channel estimationoperation whether to carry out channel smoothing on the incoming datapackets after channel estimation.

In the conventional beamforming methods, the beamformee may receive anindication from the beamformer that the transmitted beam has beenbeamformed and therefore the beamformee should not perform channelsmoothing on the data packets. Alternately, in the conventionalbeamforming methods, the beamformee may receive an indication from thebeamformer to perform channel smoothing or not perform channel smoothingon the receiving data packets and the beamformee acts accordingly.

In method 150 of the present disclosure, the beamforming matrixgenerated at the beamformee is evaluated to provide an indication ofwhether channel smoothing can be performed (step 160). In particular,the channel smoothing indicator (CHE Smoothing) is fed back to thechannel estimation operation to inform the channel estimation method toperform channel smoothing on the received data packets, regardless ofwhether the received data packets have been beamformed. The analysisstep 160 to generate the channel smoothing indicator is optional and maybe omitted in other embodiments of the present disclosure.

FIG. 5 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments. The smoothed beamforming matrixcomputation method is implemented in a beamformee to generate thebeamforming matrix which is fed back to the beamformer to enable thebeamformer to perform beamforming.

Referring to FIG. 5, a smoothed beamforming matrix computation method200 receives a channel estimation matrix H as input (202). The method200 obtains a beamforming matrix V using a beamforming algorithm andusing down-sampled subcarrier feedback indices (204). For instance, themethod 200 decomposes the channel estimation matrix H using abeamforming algorithm to obtain the beamforming matrix. In someembodiments, the method 200 decomposes the channel estimation matrix Husing SVD or GMD as the beamforming algorithm.

For example, when the method 200 uses SVD to decompose the channelestimation matrix H, the beamforming matrix V is formed by the rightsingular vectors of the decomposed channel estimation matrix H. Asdescribed above, the channel estimation matrix H has N_(tx) number ofcolumns, corresponding to N_(tx) number of transmit antennas at thebeamformer, and N_(rx) number of rows, corresponding to N_(rx) number ofreceive antennas at the beamformee. Meanwhile, the beamforming matrix Vis formed by the first N_(SS) columns of the right singular matrix ofthe decomposed channel estimation matrix having the largest singularvalues, where N_(SS) denotes the number of spatial streams for feedback.Thus, the beamforming matrix V has N_(SS) columns or N_(SS) vectors. Thebeamforming matrix V has N_(tx) number of rows, where N_(tx) is thenumber of antennas at the beamformer.

In practice, the channel estimation matrix H is computed for eachsubcarrier of the channel, each subcarrier being associated with afeedback index n. More specifically, a beamformer with N_(tx) number oftransmit antennas transmits to a beamformee with N_(rx) number ofreceive antennas N_(SS) number of spatial streams in parallel usingN_(C) number of OFDM subcarriers. Each subcarrier is identified orassociated with a feedback index “n”. In embodiments of the presentdisclosure, the method 200 forms the beamforming matrix V usingdown-sampled feedback indices. Instead of using smoothing algorithm,down-sampling is performed to reduce the possibility of brokensmoothness in the channel. In other words, instead of generatingbeamforming matrix for each subcarrier of the channel, the method 200generates a beamforming matrix for a subset of the subcarriers of thechannel associated with the down-sampled feedback index ic (204).

In some embodiments, the method 200 down-samples the feedback indices bya down-sampling ratio K. In one example, according to IEEE 802.11ac 20MHz compressed beamforming report case, there are 56 subcarriers toreport (N_(C)=56) and the method 200 uses a down-sampling ratio K=4. Thebeamforming matrix V is therefore down-sampled to 14 subcarriers usingthe down-sampling ratio of 4.

More specifically, a beamforming matrix V formed using a down-sampledsubcarrier index ic is given as:V _(ic)=[v _(1(ic)) . . . v _(NSS(ic))]where v_(ns(ic)) is a beamforming vector corresponding to ns-th spatialstream at the down-sampled subcarrier index ic. That is, thedown-sampled subcarrier index ic is a subset of the feedback index nassociated with the OFDM subcarriers of the transmit signal. Eachbeamforming vector has N_(tx) number of rows, N_(tx) being the number ofantennas at the beamformer. In one embodiment, when SVD beamformingalgorithm is used, the beamforming matrix V_(ic) is a right singularmatrix corresponding to first to N_(SS) largest singular values of thedown-sampled subcarrier's channel matrix H_(ic). The beamforming matrixV includes an array of beamforming weights where each beamforming weightis a complex weight value including an amplitude and a phase shift to beapplied for each antenna at the beamformer.

With the down-sampled beamforming matrix V_(ic) thus obtained, themethod 200 proceeds to set the last antenna's phase equals to zero androtate other antennas' phase accordingly (206). In the presentdescription, the last antenna refers to the last row index of thebeamforming matrix V. As described above, a beamforming matrix includesan array of beamforming weights where each beamforming weight is acomplex weight represented by the amplitude and the phase shift to beapplied for each antenna. According to IEEE 802.11 compressedbeamforming feedback, the phase of the beamforming weight of the lastantenna is set to zero and the phase of the beamforming weights of otherantennas is rotated with respect to a phase angle of the last antenna.In some embodiments, the antenna rotation can be implemented using knownantenna rotation algorithms.

The method 200 then performs smoothing on the beamforming feedbackmatrix thus obtained (208). Because the beamforming matrix has beendown-sampled, the method 200 performs smoothing to fill out the othersubcarriers so as to guarantee channel smoothness. In some embodiments,the method 200 performs smoothing for each antenna and each spatialstream. Furthermore, in one embodiment, the method 200 performssmoothing for each antenna and each data stream by normalizing eachcolumn vector.

In one embodiment, the method 200 generates a smoothed beamformingfeedback matrix using the original feedback granularity, that is, adown-sampled ratio of 1. Accordingly, the method 200 providesbeamforming matrix for the full set of subcarrier for feedback. Themethod 200 may further make the resultant smoothed beamforming matrixinto a unitary matrix by normalizing each column vector to unit powerand making each column vector orthogonal. In one embodiment, the method200 uses the Gram-Schmidt orthogonalization. As a result, the method 200generates a smoothed beamforming matrix V including beamforming vectorsfor all subcarriers.

The method 200 then compresses or quantizes the smoothed beamformingfeedback matrix (210). In embodiments of the present disclosure, themethod 200 applies compression or quantization techniques known in theart. The compressed beamforming matrix can then be transmitted to thebeamformer (212).

In the present embodiment, the method 200 further evaluates thecompressed beamforming matrix and determines whether channel smoothingcan be performed for the data packets even if the beamformee receivesbeamformed data packets. The method 200 evaluates a metric for thebeamforming matrix and determines if the metric is greater than apredetermined threshold (214). In one embodiment, the metric can be thesign-to-noise ratio of the beamforming matrix. In another embodiment,the metric can be a correlation value. In another embodiment, the metriccan be the number of spatial streams for feedback.

In embodiments of the present disclosure, the method 200 uses theevaluation of the metric to set a channel smoothing indicator which iscoupled to the channel estimation operation. The channel smoothingindicator is fed back to the channel estimation operation to instructthe channel estimation method to perform channel smoothing on thereceived data packets, regardless of whether the received data packetshave been beamformed. In one embodiment, if the beamforming matrixmetric exceeds a given threshold, the channel smoothing indicator is setto On (216). On the other hand, if the metric computed for thebeamforming matrix is at or below the threshold, the channel smoothingindicator is set to Off (218). The method 200 then provides the channelsmoothing indicator to the channel estimation operation (220). Thechannel estimation operation will be configured to perform channelsmoothing for incoming data packets when the channel smoothing indicatoris set to On. The channel estimation operation will be configured toperform channel smoothing according to information provided by thebeamformer when the channel smoothing indicator is set to Off. That is,when the channel smoothing indicator is set to Off, then the channelestimation will not perform channel smoothing when the beamformerindicates to the beamformee that the transmitted data packet has beenbeamformed and will perform channel smoothing when the beamformerindicates to the beamformee that the transmitted data packet has notbeen beamformed.

The method 200 completes the computation of the beamforming matrix andthe method 200 may be repeated when another channel estimation matrix isreceived from the channel estimation operation.

In some embodiments, the metric of the beamforming matrix can becompared with a threshold having the opposite polarity—that is, channelsmoothing indicator is set to On in response to the metric being equalto or small than the threshold and to Off in response to the metricbeing greater than the threshold. In embodiments of the presentdisclosure, the threshold used for the beamforming matrix metric canhave variable values. In particular, different threshold may be used fordifferent SNR region. The method 200 in FIG. 5 is illustrative only andnot intended to be limiting.

In one embodiments, the beamforming matrix metric is a correlation valueof the beamforming weights between neighboring subcarriers. In thepresent embodiment, the correlation value is computed between thebeamforming weights of neighboring down-sampled subcarriers. Aftercompression, if the minimum correlation

$\left( {\min\limits_{{\forall{n\; s}},{\forall{i\; c}}}{{v_{{n\; s},{{i\; c} - 1}}^{H}v_{{n\; s},{ic}}}}} \right)$is smaller than a threshold or the maximum phase angle

$\left( {\min\limits_{{\forall{n\; s}},{\forall{i\; c}}}{{\measuredangle v}_{{n\; s},{{i\; c} - 1}}^{H}v_{{n\; s},{ic}}}} \right)$is larger than a threshold (214), then method 200 set the channelsmoothing indicator (CHE Smoothing) to off (218). Otherwise, the method200 set the channel smoothing indicator (CHE Smoothing) to on (216). Thechannel smoothing indicator (CHE Smoothing) is stored locally at thebeamformee to use in its own channel estimation operation.

In embodiments of the present disclosure, the smoothed beamformingmatrix computation method 200 generates the channel smoothing indicatorto instruct the channel estimation operation to carry out channelsmoothing if certain requirement for channel smoothing is met. Thus, thedecision to carry out channel smoothing or not is decoupled from theinstruction from the beamformer. Even if the beamformer indicates thetransmitted packets have been beamformed, the beamformee may stillperform channel smoothing after channel estimation when the conditionsevaluated by the metric are met. In this manner, optimal performance canbe obtained at the beamformee.

In particular, setting the channel smoothing indicator locally based onthe beamforming matrix evaluation realizes advantages over conventionalsolutions. First, without the use of thresholds, the beamformee may haveto apply channel smoothing all the time. This will degrade performancewhen channel smoothing should not be applied under certain conditions.Second, if channel smoothing is not applied at all, then performance isalso degraded. The channel smoothing indicator enables the beamformee toapply channel smoothing intelligently so that channel smoothing isapplied when channel smoothing will improve the performance of thewireless station.

FIG. 6 is a flowchart illustrating a smoothed beamforming matrixcomputation method in some embodiments. The smoothed beamforming matrixcomputation method is implemented in a beamformee to generate thebeamforming matrix which is fed back to the beamformer to enable thebeamformer to perform beamforming. The smoothed beamforming matrixcomputation method is similar to the method 200 in FIG. 5 and similarsteps will not be further described. In particular, the smoothedbeamforming matrix computation method of FIG. 6 includes the additionalstep of performing vector swapping and also uses an inventive method toset the phase of the last antenna to zero, as will be described in moredetail below.

Referring to FIG. 6, a smoothed beamforming matrix computation method250 receives a channel estimation matrix H as input (252). The method200 obtains a beamforming matrix V using a beamforming algorithm andusing down-sampled subcarrier feedback indices (254). As describedabove, the method 250 may decompose the channel estimation matrix Husing a beamforming algorithm, such as SVD, to obtain the beamformingmatrix. The method 250 down-samples the subcarrier feedback indices by adown-sampling ratio K. That is, instead of generating beamforming matrixfor each subcarrier of the channel, the method 200 generates abeamforming matrix for a subset of the subcarriers of the channel.

The method 250 then performs vector swapping (256). As described above,channel smoothness can be affected when the feedback vectors for twosubcarriers are swapped. In embodiments of the present disclosure,vector swapping is carried out to guarantee higher correlation betweensubcarriers for the same spatial stream. Vector swapping is applied inthe case when there are more than one spatial stream, that is N_(SS)>1.In the case the number of spatial streams is greater than 1, the method250 computes the correlation between neighboring down-sampledsubcarriers across all the spatial streams. For each subcarrier ic, themethod 250 then places column vector of the beamforming matrix whichshows the largest correlation with respect to column vector of theneighbor down-sampled subcarrier having index ic−1. In the conventionalmethod, for each subcarrier, the beamforming matrix usually arranges thefeedback vectors so that the vector with the largest singular value islisted first. However, in embodiments of the present disclosure, themethod 250 evaluates the correlation of a column vector of onesubcarrier relative to the column vector of a neighbor down-sampledsubcarrier and arranges the column vector based on the correlationbetween adjacent down-sampled subcarriers. FIG. 7 illustrates apseudocode which can be used to implement the vector swapping operationin some embodiments of the present disclosure.

In one example, the beamforming matrix includes two neighboringsubcarriers SC1 and SC2. The beamforming matrix for subcarrier SC1 maybe [V1, V2, V3] in the order of decreasing singular values while thebeamforming matrix for SC2 may be [V2, V1, V3] in the order ofdecreasing singular values. That is, subcarrier SC1 has vector V1 havingthe largest singular value while subcarrier SC2 has vector V2 having thelargest singular value. In this case, channel smoothness may be brokendue to the vector swapping between the two neighboring subcarriers.However, in embodiments of the present disclosure, the method 250applies vector swapping operation to arrange the feedback vectors basedon correlation between neighboring subcarriers. Accordingly, the method250 will generate beamforming matrices for subcarriers SC1 and SC2 thatare both [V1, V2, V3]. In this manner, channel smoothness can bemaintained.

With the down-sampled beamforming matrix V_(ic) thus obtained and withvector swapping applied, the method 250 proceeds to set the lastantenna's phase equals to zero and rotate other antennas' phaseaccordingly (258). In the present description, the last antenna refersto the last row the beamforming matrix. The method 250 applies a noveltechnique to set the last antenna's phase to zero. In particular, themethod 250 considers the magnitude or amplitude of the beamformingweight of the last antenna. In the case that the amplitude of thebeamforming weight is small, that is, the amplitude of beamformingweight of the last antenna is less than a weight threshold, the method250 aligns the phase of all the antennas to the phase associated withthe antenna having the largest amplitude in the beamforming weight. Themethod 250 then set the last antenna's phase value to zero by settingthe phase value of the last antenna to zero without further rotating theother antenna's phase values.

As described above, a beamforming matrix includes an array ofbeamforming weights where each beamforming weight is a complex weightrepresented by an amplitude and a phase to be applied for each antenna.In the present description, references to the “beamforming weight” referto the amplitude of the beamforming weight with the understanding thatthe beamforming weight also has associated with it a phase angle.Furthermore, in the present description, the amplitude of thebeamforming weight is sometimes referred to as the magnitude of thebeamforming weight.

In embodiments of the present disclosure, the method 250 evaluates theamplitude or magnitude of the last antenna's beamforming weight todetermine the phase rotation of the beamforming weights of the antennasto set the last antenna's phase to zero. Furthermore, in someembodiments, the method 250 further evaluates the correlation of thebeamforming vectors between neighboring down-sampled subcarriers todetermine the phase rotation of the beamforming weights of the antennasto set the last antenna's phase to zero.

In one embodiment, suppose k* is an index of the maximum absolutebeamforming weight value, or the maximum absolute amplitude of thebeamforming weight, that is:

${k^{*} = {\max\limits_{k = {1\mspace{14mu}\ldots\mspace{14mu} N_{tx}}}{{v_{{ns},{ic}}(k)}}}},$where v_(ns,ic)(k) is k's element of vector v_(ns,ic), and the lastantenna is denoted by N_(tx).

The method 250 evaluates two conditions to determine how to rotate thephase angle of the beamforming weights for all the antennas. First, themethod 250 determines if the correlation value between feedback vectorsof neighboring down-sampled subcarriers is greater than a giventhreshold value (e.g. 0.9 or 0.95). Second, the method 250 determines ifthe amplitude of the last antenna is smaller than a given thresholdvalue (e.g. 0.1).

More specifically, if the correlation value of the beamforming vectorsbetween neighboring down-sampled subcarriers (Corr=|v_(ns,ic-1) ^(H)v_(ns,ic)|) and last antenna's magnitude meet the requirement (forexample, Corr>0.95 or Corr>0.9 and |v_(ns,ic)(N_(tx))|<0.1), that is, ifthe correlation value is high while the amplitude of the last antenna'sbeamforming weight is small, then method 250 rotates all antennas'beamforming weight with respect to the angle difference between thecurrent subcarrier and previous down-sampled subcarrier's phase anglefor the k* element, that is, the antenna element having the maximumabsolute beamforming weight value. Accordingly:v _(ns,ic) =e ^(j·θ) v _(ns,ic), where 0=

v _(ns,ic-1)(k*)−

v _(ns,ic)(k*).Then, the method 250 set the phase of the last antenna (N_(tx)) to zeroby:v _(ns,ic)(N _(tx))=|real(v _(ns,ic)(N _(tx)))| where real(x) is realvalue of x.In some embodiments, the last antenna's phase can be set by:v _(ns,ic)(N _(tx))=|v _(ns,ic)(N _(tx))|.

If both of the above conditions are not met, that is, the correlationvalue between neighboring down-sampled subcarriers is not large enoughand the amplitude of the last antenna's beamforming weight is not toosmall, then the method 250 rotates all antennas' beamforming weight withrespect to the phase angle of last antenna, that is:v _(ns,ic) =e ^(jθ) v _(ns,ic),θ=

_(ns,ic)(N _(tx)).

More specifically, when the amplitude of the last antenna's beamformingweight is small, rotating based on the last antenna's phase may resultin phase discontinuity, thus breaking channel smoothness. In embodimentsof the present disclosure, when the amplitude of the last antenna'sbeamforming weight is small while the correlation of the beamformingvector between neighboring down-sampled subcarriers is high, the method250 uses the phase of the beamforming weight with the maximum amplitudeas a reference phase. The method 250 rotates the phase of all theantennas using the reference phase. As a result, the phase of all theantennas is aligned with the reference phase, being the phase of thebeamforming weight having the largest amplitude. After rotation usingthe reference phase, if the phase of the last antenna is not zero, themethod 250 forces the phase of the last antenna (N_(tx)) to zero bysetting the beamforming weight to the real part (amplitude) of the lastantenna's beamforming weight or by taking the magnitude of the lastantenna's beamforming weight.

In the case the amplitude of the last antenna's beamforming weight isnot too small or while the correlation between two neighboringdown-sampled subcarrier beamforming vectors is not high enough, themethod 250 rotates all antennas' beamforming weight with respect to thephase angle of last antenna to set the last antenna's phase to zero.

In the present embodiment, the amplitude of the last antenna'sbeamforming weight and the correlation value between neighboringdown-sampled subcarriers are used to determine the phase rotationmethods to use. In other embodiments, the amplitude of the lastantenna's beamforming weight or the correlation value betweenneighboring down-sampled subcarriers can be used to determine the phaserotation method to apply.

The method 250 then performs smoothing on the beamforming feedbackmatrix thus obtained (260). Because the feedback indices of thebeamforming matrix has been down-sampled, the method 250 performssmoothing to fill out the other subcarriers so as to guarantee channelsmoothness. In some embodiments, the method 250 performs smoothing foreach antenna and each spatial stream. In one embodiment, the method 250generates a smoothed beamforming feedback matrix using the originalfeedback granularity, that is, a down-sampled ratio of 1. That is, thefeedback matrix for all the subcarriers are generated. The method 250may further make the resultant smoothed beamforming matrix into aunitary matrix by normalizing each column vector to unit power andmaking each column vector orthogonal. In one embodiment, the method 250uses the Gram-Schmidt orthogonalization. As a result, the method 250generates a smoothed beamforming matrix V including beamforming vectorsfor all subcarriers.

The method 250 then compresses or quantizes the smoothed beamformingfeedback matrix (262). In embodiments of the present disclosure, themethod 250 applies compression or quantization techniques known in theart. The compressed beamforming matrix can then be transmitted to thebeamformer (264).

The method 250 further evaluates the compressed beamforming matrix anddetermines whether channel smoothing can be performed even if thebeamformee receives beamformed data packets. The method 250 evaluates ametric for the beamforming matrix and determines if the metric isgreater than a predetermined threshold (266). In one embodiment, themetric can be the sign-to-noise ratio of the beamforming matrix. Inanother embodiment, the metric can be a correlation value betweenneighboring subcarriers. In another embodiment, the metric is acombination of the signal-to-noise ratio and a correlation value. Inother embodiments, the metric is the number of spatial streams forfeedback.

In embodiments of the present disclosure, the method 250 uses theevaluation of the metric to set a channel smoothing indicator which iscoupled to the channel estimation operation. The channel smoothingindicator is fed back to the channel estimation operation to instructthe channel estimation method to perform channel smoothing on thereceived data packets, regardless of whether the received data packetshave been beamformed. In one embodiment, if the beamforming matrixmetric exceeds a given threshold, the channel smoothing indicator is setto On (268). On the other hand, if the metric computed for thebeamforming matrix is at or below the threshold, the channel smoothingindicator is set to Off (270). The method 250 then provides the channelsmoothing indicator to the channel estimation operation (272). Thechannel estimation operation will be configured to perform channelsmoothing when the channel smoothing indicator is set to On. The channelestimation operation will be configured to perform channel smoothingaccording to information provided by the beamformer when the channelsmoothing indicator is set to Off. That is, when the channel smoothingindicator is set to Off, then the channel estimation will not performchannel smoothing when the beamformer indicates to the beamformee thatthe transmitted data packet has been beamformed and will perform channelsmoothing when the beamformer indicates to the beamformee that thetransmitted data packet has not been beamformed.

The method 250 completes the computation of the beamforming matrix andthe method 250 may be repeated when another channel estimation matrix isreceived from the channel estimation operation.

In some embodiments, the metric of the beamforming matrix can becompared with a threshold having the opposite polarity—that is, channelsmoothing indicator is set to On in response to the metric being equalto or small than the threshold and to Off in response to the metricbeing greater than the threshold. Furthermore, the metric may be acombination of factors and evaluated using various thresholds. Inembodiments of the present disclosure, the threshold used for thebeamforming matrix metric can have variable values. In particular,different thresholds may be used for different SNR region. The method250 in FIG. 6 is illustrative only and not intended to be limiting.

The method 250 in FIG. 6 introduces additional techniques to ensure asmoothed beamforming matrix is generated to guarantee channelsmoothness. In particular, the method 250 uses vector swapping andweight-based phase rotation. In other embodiments, one or both of thetechniques can be applied to improve the smoothness of the beamformingmatrix. That is, vector swapping can be applied independent of theweight-based phase rotation, and vice versa. The method 250 in FIG. 6incorporating both techniques is illustrative only.

The present disclosure can be implemented in numerous ways, including asa process; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a hardware processor or a processor device configuredto execute instructions stored on and/or provided by a memory coupled tothe processor. In this specification, these implementations, or anyother form that the present disclosure may take, may be referred to astechniques. In general, the order of the steps of disclosed processesmay be altered within the scope of the present disclosure. Unless statedotherwise, a component such as a processor or a memory described asbeing configured to perform a task may be implemented as a generalcomponent that is temporarily configured to perform the task at a giventime or a specific component that is manufactured to perform the task.As used herein, the term ‘processor’ refers to one or more devices,circuits, and/or processing cores configured to process data, such ascomputer program instructions.

A detailed description of one or more embodiments of the presentdisclosure is provided below along with accompanying figures thatillustrate the principles of the present disclosure. The presentdisclosure is described in connection with such embodiments, but thepresent disclosure is not limited to any embodiment. The scope of thepresent disclosure is limited only by the claims and the presentdisclosure encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of the presentdisclosure. These details are provided for the purpose of example andthe present disclosure may be practiced according to the claims withoutsome or all of these specific details. For the purpose of clarity,technical material that is known in the technical fields related to thepresent disclosure has not been described in detail so that the presentdisclosure is not unnecessarily obscured.

The above detailed descriptions are provided to illustrate specificembodiments of the present disclosure and are not intended to belimiting. Numerous modifications and variations within the scope of thepresent disclosure are possible. The present disclosure is defined bythe appended claims.

What is claimed is:
 1. A method comprising: receiving a channelestimation matrix characterizing a communication channel between awireless station and an access point, the channel estimation matrixbeing associated with a plurality of feedback indices relating to aplurality of subcarriers; determining a beamforming matrix using thechannel estimation matrix, the beamforming matrix being generated for aplurality of down-sampled feedback indices being a subset of theplurality of feedback indices; the beamforming matrix includingbeamforming weights for one or more antennas among a plurality ofantennas associated with the access point, each beamforming weight beinga complex value including an amplitude and a phase; setting the phase ofthe beamforming weight for a last antenna associated with the last rowindex of the beamforming matrix to zero and rotating the phase of eachbeamforming weight for each of the other antennas; performing smoothingof the beamforming matrix for each feedback index by normalizing eachcolumn vector of the beamforming matrix; compressing the smoothedbeamforming matrix; determining to selectively apply channel smoothingto the channel estimation matrix by evaluating the compressed smoothedbeamforming matrix based on a metric of the beamforming matrix; andproviding the smoothed beamforming matrix as output.
 2. The method ofclaim 1, wherein providing the smoothed beamforming matrix as outputcomprises: transmitting the smoothed beamforming matrix to the accesspoint.
 3. The method of claim 1, wherein determining, at the wirelessstation, the beamforming matrix comprises: decomposing the channelestimation matrix using a beamforming algorithm.
 4. The method of claim3, wherein decomposing the channel estimation matrix using a beamformingalgorithm comprises: decomposing the channel estimation matrix usingsingular value decomposition, wherein the beamforming matrix is formedby the right singular vectors of the decomposed channel estimationmatrix.
 5. The method of claim 3, wherein decomposing the channelestimation matrix using a beamforming algorithm comprises: decomposingthe channel estimation matrix using geometric mean decomposition.
 6. Themethod of claim 1, further comprising: determining a first phase valueof the beamforming weight for the last antenna; rotating the phase ofeach beamforming weight of each antenna using the first phase of thebeamforming weight for the last antenna; and setting the phase of thebeamforming weight for the last antenna to zero as a result of therotating.
 7. The method of claim 1, further comprising: determining anamplitude of the beamforming weight for the last antenna; in response tothe amplitude being less than a first threshold, determining a secondphase value of a beamforming weight having a maximum amplitude among thebeamforming weights of the plurality of antennas; rotating the phase ofeach of the beamforming weight of the plurality of antennas except thelast antenna using the second phase value; and setting the phase of thebeamforming weight for the last antenna to zero.
 8. The method of claim1, further comprising: determining a correlation value betweenbeamforming matrices associated with neighboring down-sampledsubcarriers; in response to the correlation value being greater than asecond threshold, determining a second phase value of a beamformingweight having a maximum amplitude among the beamforming weights of theplurality of antennas; rotating the phase of each of the beamformingweight of the plurality of antennas except the last antenna using thesecond phase value; and setting the phase of the beamforming weight forthe last antenna to zero.
 9. The method of claim 1, further comprising:determining correlation values of beamforming matrices betweenneighboring down-sampled subcarriers; arranging column vectors of thebeamforming matrix of a first subcarrier based on the determining, thecolumn vectors being arranged based on the correlation values betweenthe neighboring down-sampled subcarriers.
 10. The method of claim 1,further comprising: in response to the metric being in a first range,setting the channel smoothing indicator to on; providing the channelsmoothing indicator to apply the channel smoothing to the channelestimation matrix; in response to the metric being in a second range,setting a channel smoothing indicator to off; and providing the channelsmoothing indicator to not apply the channel smoothing to the channelestimation matrix.
 11. The method of claim 1, wherein the metric of thebeamforming matrix includes a signal-to-noise ratio of the beamformingmatrix.
 12. The method of claim 1, wherein the metric of the beamformingmatrix includes a correlation value of beamforming vectors betweenneighboring subcarriers in the beamforming matrix.
 13. The method ofclaim 1, wherein the metric of the beamforming matrix includes a numberof spatial streams for feedback as the metric.
 14. A wireless stationcomprising: a processor; and a memory coupled with the processor,wherein the memory is configured to provide the processor withinstructions which when executed cause the processor to: receive achannel estimation matrix characterizing a communication channel betweenthe wireless station and an access point, the channel estimation matrixbeing associated with a plurality of feedback indices relating to aplurality of subcarriers; determine a beamforming matrix using thechannel estimation matrix, the beamforming matrix being generated for aplurality of down-sampled feedback indices being a subset of theplurality of feedback indices; the beamforming matrix includingbeamforming weights for one or more antennas among a plurality ofantennas associated with the access point, each beamforming weight beinga complex value including an amplitude and a phase; set the phase of thebeamforming weight for a last antenna associated with the last row indexof the beamforming matrix to zero and rotating the phase of eachbeamforming weight for each of the other antennas; perform smoothing ofthe beamforming matrix for each feedback index by normalizing eachcolumn vector of the beamforming matrix; compress the smoothedbeamforming matrix; determine to selectively apply channel smoothing tothe channel estimation matrix by evaluating the compressed smoothedbeamforming matrix based on a metric of the beamforming matrix; andprovide the smoothed beamforming matrix as output.
 15. The wirelessstation of claim 14, wherein the memory is further configured to providethe processor with instructions which when executed cause the processorto: transmit the smoothed beamforming matrix to the access point. 16.The wireless station of claim 14, wherein the memory is furtherconfigured to provide the processor with instructions which whenexecuted cause the processor to: decompose the channel estimation matrixusing singular value decomposition, wherein the beamforming matrix isformed by the right singular vectors of the decomposed channelestimation matrix.
 17. The wireless station of claim 14, wherein thememory is further configured to provide the processor with instructionswhich when executed cause the processor to: determine an amplitude ofthe beamforming weight for the last antenna; in response to theamplitude being less than a first threshold, determine a second phasevalue of a beamforming weight having a maximum amplitude among thebeamforming weights of the plurality of antennas; rotate the phase ofeach of the beamforming weight of the plurality of antennas except thelast antenna using the second phase value; and set the phase of thebeamforming weight for the last antenna to zero.
 18. The wirelessstation of claim 14, wherein the memory is further configured to providethe processor with instructions which when executed cause the processorto: determine a correlation value between beamforming matricesassociated with neighboring down-sampled subcarriers; in response to thecorrelation value being greater than a second threshold, determine asecond phase value of a beamforming weight having a maximum amplitudeamong the beamforming weights of the plurality of antennas; rotate thephase of each of the beamforming weight of the plurality of antennasexcept the last antenna based on the second phase value; and set thephase of the beamforming weight for the last antenna to zero.
 19. Thewireless station of claim 14, wherein the memory is further configuredto provide the processor with instructions which when executed cause theprocessor to: in response to the metric being in a first range, set thechannel smoothing indicator to on; provide the channel smoothingindicator to apply the channel smoothing to the channel estimationmatrix; in response to the metric being in a second range, set a channelsmoothing indicator to off; and provide the channel smoothing indicatorto not apply the channel smoothing to the channel estimation matrix. 20.The wireless station of claim 19, wherein the metric of the beamformingmatrix includes a signal-to-noise ratio of the beamforming matrix.